This film became the biggest box office hit of director Kurt Neumann's career, but he never knew it or even found out about it. He died one month after the film's premiere and only one week before it went into general release.
Part of the laboratory set used in this film was Emerac, the computer from Fox's production Une femme de tête (1957).
Although many people who saw this film swear it was made in black and white, it never was. This may be an example of the "Mandela Effect", which is simply a name for a collective false memory among people which never really happened and yet everyone remembers. It is extremely common. La Mouche noire (1958) was only ever filmed and shown in color; however, its two sequels, Le Retour de la mouche (1959) and La malédiction de la mouche (1965), were made in black and white. This is likely where the confusion comes from. Another confusing aspect is that when the first Fly film was referenced in books and articles, promotional stills from The Return of the Fly were frequently used, so many people thought that the black & white pictures from Return actually belonged to the first film.
David Hedison suggested that his character of Andre Delambre wear progressive makeup effects that showed him in a combined part human/part fly state when his face is finally revealed instead of just a fully formed fly head as depicted in the film. The producers declined due to the cost and time required to achieve the combined part human/part fly look, as well as it being too harsh and grotesque for audiences at the time. This concept was later embraced completely in the 1986 remake, which focused on the scientist devolving slowly into a mutated part human/part fly creature.
Michael Rennie was offered the role of Andre Delambre in the film, but turned it down because his head would be covered throughout most of it.